1. Field of the Invention
The present invention relates to a process for the production of alkyl aromatics such as alkyl benzene and, more specifically, to an improved method for recovering alkyl aromatics from an alkylation reaction.
2. Description of the Prior Art
The alkylation of aromatic hydrocarbons to produce alkyl aromatic hydrocarbons is well known. Aromatic hydrocarbons can be alkylated by suitable reaction with alkylating agents such as alkenes, halo alkanes, etc., the reaction normally being carried out in the presence of acidic catalysts, such as aluminum chloride, hydrogen fluoride or the like.
The alkyl aromatic compounds produced find widespread uses in many industrial applications. In particular, alkyl aromatic compounds, such as alkyl benzenes, wherein the alkyl group has a chain length of from about 9 to about 14 carbon atoms are particularly useful in producing detergents. Such alkyl benzenes are sulfonated using oleum or sulfur trioxide to produce an alkyl benzene sulfonate, a widely used detergent.
One of the most critical quality parameters for an alkyl benzene sulfonate detergent is the color characteristic. The detergent manufacturing industry generally requires that the alkyl aromatic be a water white product and which upon sulfonation with SO.sub.3 or oleum yields light colored sulfonate salts. It is generally believed that alkyl benzenes which are off spec in producing unacceptable dark sulfonate salts contain polynuclear aromatic by-products which, when sulfonated, are yellow to black in color and produce very dark colored sulfonates. It is also known that the sulfonate color precursors, such as naphthalene and anthracene type compounds, have boiling points ranging throughout the alkyl aromatic boiling range, making complete separation by distillation virtually impossible. Accordingly, to meet the essentially pure, water white color, it is common to purge part of the alkyl aromatic fraction from the system, i.e. the tail end or higher boiling point of the mono alkyl aromatic product leaves the process to be sold as a lower value by-product or used for fuel, in an attempt to purge the color precursors from the product. This results in only 97.5 to about 98.5% of the available mono alkyl aromatic in the reaction product being recovered. Even with this expedient, which results in loss of the valuable product mono alkyl aromatic, in order to meet the minimum purity standards required by the detergent manufacturing industry, the recovered mono alkyl aromatic must be further refined with a sulfuric acid wash and contacted with a suitable, basic absorbent such as an Attapulgus clay.